Hafnium nanoferrite boost for memory device power

October 24, 2018 // By Nick Flaherty
The 'Hafnia team' within the Nanostructures of Functional Oxides group, Zernike Institute for Advanced Materials, University of Groningen. From left to right: Yingfen Wei, Pavan Nukala, Beatriz Noheda, and Mart Salverda.  CREDIT Henk Bonder, University of Groningen
Dutch researchers at the University of Groningen have demonstrated that tiny crystals of hafnium oxide show ferroelectric properties, allowing them to be used for memory devices that need dramatically less power.

Ferroelectric materials have a spontaneous dipole moment which can point up or down. This means that they can be used to store information, just like magnetic bits on a hard disk. The advantage of ferroelectric memory bits is that they can be written at a low voltage and power, while magnetic bits require large currents to create a magnetic field for switching, and so need more power. The disadvantage of ferroelectrics is that the aligned dipoles are only stable in fairly large groups, so as the size of the crystals reduces, the dipole moment eventually disappears.

"Reducing the size of ferroelectric materials has been a research topic for more than 20 years" said Prof Beatriz Noheda, Professor of Functional Nanomaterials at UG. "Some eight years ago, a breakthrough was announced by the Nanoelectronic Materials Laboratory in Dresden, Germany. They claimed that hafnium oxide thin films were ferroelectric when thinner than ten nanometres and that thicker films actually lost their ferroelectric properties. This went against everything we knew, so most scientists were skeptical, including me," she said. 

Some of the skepticism was because the ferroelectric hafnium samples used in these studies were polycrystalline and showed multiple phases, obscuring any clear fundamental understanding of such an unconventional phenomenon.

Noheda and her group wanted to study these crystals by growing clean (single-phase) films on a substrate. Using X-ray scattering and high-resolution electron microscopy techniques, they observed that thin films under 10nm thick grow in an entirely unexpected and previously unknown polar structure, which is necessary for ferroelectricity. Combining these observations with transport measurements, they confirmed that the material was indeed ferroelectric. "In the substrate that we used, the atoms were a little bit closer than those in hafnium oxide, so the hafnium crystals would be a little strained",  said Prof Noheda. 

A key discovery was that the crystal structure changed when the layers exceeded 10nm, reproducing the results of the Dresden lab as a result. "We

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